More than 500 people have flown in space. But they’re far from the only living things to leave our planet. Everything from gnats to chimpanzees have also taken the ride to orbit.

At first, these non-human passengers helped scientists assess the risks that humans would face as they ventured beyond Earth — the crushing force of acceleration, the uncertain challenges of zero gravity, and the cascade of radiation from the Sun and beyond.

The first living thing in orbit was Laika, a dog launched by the Soviet Union in late 1957. The pooch died when the capsule overheated soon after launch. Several other dogs soon flew in space as well, and so did a rabbit.

The United States sent a chimpanzee, named Ham, into space in 1961, paving the way for the Mercury astronauts to follow.

Since those early days, many other living things have flown in space as parts of experiments in basic biology. The list of subjects has included mice, fruit flies, jellyfish, frogs, newts, and many others.

A few years ago, European scientists flew an experiment known as TARDIS — Tardigrades in Space. Tardigrades are able to adapt to just about anything. When they’re exposed to harsh conditions they enter a state that’s like hibernation. When conditions improve, they re-animate. The little critters of TARDIS survived several days of direct exposure to the vacuum and radiation of space.

Tardigrades are also being used in experiments that simulate conditions on Mars. More about that tomorrow.